There is a growing interest in electrical energy metering in support of energy efficiency and conservation as well as bill reduction. Electrical power meters for both temporary and permanent installations have been used. Electrical power measurement often requires simultaneous measurement of both voltage and current (often using a sensor called a current transformer (“CT”) as a sensor) on one, two, or three electrical phases. Inside the meter the voltage is multiplied by the current (from the CT) to calculate watts, kilowatt-hours, and other electrical parameters.
For two-phase and three-phase systems to be metered correctly, the phase of the wire the current sensor is placed on must match or correspond to the phase of the voltage that is measured and multiplied by the current to calculate watts.
For a typical commercial or industrial electrical service, there are three phases: L1, L2, and L3 (though different designators can also be used). When metering a three-phase service the meter will be connected to the three phase wires and a CT placed around each phase wire as well. Correct measurement requires that the L1 current measured by the CT placed on the L1 phase wire be internally (to the meter) multiplied by the L1 voltage measured by the voltage measured on the L1 phase wire. The same is true for the L2 & L3 phases. A three-phase system usually includes four wires: one wire for each phase and a neutral wire. Measuring the relative voltages requires four voltage probes, connected to each wire, while measuring currents requires only three current probes, one connected to each of the three “hot” wires. For delta loads, the system does not include a neutral wire, whereby power is transferred via the “delta” (or relative difference) between each of the three phases. (Both 3 wire and 4 wire systems usually have a ground wire that is for electrical safety but is not part of the measurement circuit.)